The long term objective of this work is to understand the functional architecture of cortical systems in mammals, from both a biophysical and a computational point of view. Although the hippocampal formation is recognized as a structure involved in memory formation and storage, the cellular and network mechanisms underlying its functional architecture remain unsolved. This issue will be confronted by an examination of the biophysical properties of synaptic interaction between principal cells and local circuit neurons in the fascia dentata subfield of rat hippocampus. Specifically, changes in synaptic function (such as the associative long- term enhancement which results from electrical stimulation) will be measured, using in vitro and intracellular recording and staining techniques, with the goal of quantifying the plastic synaptic interactions that control the operating characteristics of this network. The role of interneurons, which has to a large extent been neglected, will be a specific focus of attention with respect to (1) the dynamic control they exert over information processing in this tissue, and (2) their role in depotentiation, or 'unlearning' effects. It is expected that these studies of hippocampal neurophysiology will eventually expand to include cerebral cortex, and other areas of the brain involved in mnemonic function. The information gained from these studies will provide new insight into normal brain mechanisms, as well as the mechanisms that underly brain disorders linked to abnormal neural organization, such as schizophrenia, dementia, and alzheimer's disease.